In most electromagnetic systems, the transfer of energy from one component of the system to another is critical to proper operation of the system. In many electromagnetic systems, this transfer of energy is accomplished by appropriately energizing one component of the system to establish a magnetic flux that interacts with another component of the system to transfer energy from the energized component to the other component. Despite the fact that the energy transfer is accomplished by the flux, in known electromagnetic systems, the flux of the system is not directly controlled. Instead, the current and/or voltage applied to the energized member is controlled and, based on assumed relationships between current, voltage and flux, it is assumed that the control of the current and/or voltage based on the assumed relationships will produce the appropriate flux. This control of current and/or voltage is typically implemented because, to date, the prior art has not provided an efficient, low cost, and easily-implemented system for directly controlling flux in an electromagnetic system.
One drawback of current and/or voltage control systems as described above is that the relationships between current, voltage and flux are not easily represented mathematically and vary in a non-linear manner depending on a variety of variables. For example, the particular characteristics of each piece of magnetic material in a system will result in voltage, current and flux relationships that vary from one system to another and, even within a given system, from one section of the system to another. Because of these differing voltage, current and flux relationships, it is difficult to accurately and properly control the currents and/or voltages to produce the desired flux and, thus, the desired energy transfer. As such, the prior art is limited in its ability to provide an electromagnetic system in which flux is directly controlled.
The lack of an appropriate flux control system in the prior art is particularly noticeable in electromagnetic systems where the force exerted by one component of the system on another component of the system is desired to be finally controlled. In such systems, the actual force produced by the system is related to the flux established by the energized component of the system. As described above, however, because the prior art cannot directly and finely control flux, it cannot, therefore, finely control the force produced by such systems.
The inability of the prior art to finely control the forces established in an electromagnetic system is particularly acute in applications where the movement of at least one component of the system must be precisely controlled. One example of such an application is in a friction or vibration welder for driving a thermoplastic part to be welded with linear, orbital, rotational, or arbitrary vibratory motion relative to another thermoplastic part with the two parts in forced contact along surfaces thereof to be welded such that the relative movement of the parts relative to one another causes friction to heat the parts along the intersections thereof so that upon cessation of the movement, the parts will cool and will be welded to one another.
Friction welders are especially suitable for use in the welding of thermoplastic parts by means of either linear, spin, or orbital vibratory forces which induce friction heating in the parts. This friction heating at the interface of the surfaces to be welded causes the parts to fuse at their interface surfaces and bond together as they cool. Although the vibratory forces generating the friction heating may be created through mechanical coupling means, it is common to employ an electromagnetic system to generate the controlled motion necessary.
Numerous friction welders driven either electromagnetically or hydraulically are commercially available for operating in a linear vibratory mode. However, the motion of these friction welders is not ideal. Due to the linear or side-to-side motion of the welding component, the frictional forces at the interface of the welding materials are translational, and drop to a zero velocity each time the welding components reverse direction. When the components are at zero velocity, no heat is produced, as friction welding is a resistance process, producing heat proportional to the product of the resistance forces and the mean-squared relative velocity of the components at the interface.
Furthermore, many linear motion welding systems employ electromagnetic systems or drivers using the known "Scott Tee" magnetic circuit to convert three-phase electrical power into one-phase mechanical motion. In such systems, because of the electromagnetically-linked nature of the driver component of the system, it is difficult to precisely control movement of the movable member in all directions and to limit zero velocity intervals for the movable member. Accordingly alternative motions and controllers for frictional welding components have been developed which seek to reduce or minimize the zero velocity components and simplify the control circuits.
Spin welding is one such alternative in which the welding components are spun about an axis, and rotational forces, not linear motion, produce the frictional heating at the interface. However, the rotational forces are proportional to the radial distance from the center of rotation, and accordingly neither the velocity of the components nor the resulting heating is radially uniform. Furthermore, spin welding is generally restricted to applications where the parts to be welded have circular geometry.
A second alternative is to electromagnetically produce an orbital motion of the welding components. During orbital motion, the velocity of the components remains constant as the parts are rubbed, thereby generating the same amount of frictional heating as is generated by linear motion, but with less required force and less relative displacement of the welding components.
Despite the heating advantages of orbital welding, there are parts which are not amenable to welding with orbital motion, but are amenable to welding by either linear or spin motion. Accordingly, friction welders which are electromagnetically driven and capable of producing either linear motion or orbital motion have been developed. One such friction welder is disclosed in Snyder, U.S. Pat. No. 5,378,951. The electromagnetic drive system of these friction welders is in several ways similar to that for an electromagnetic motor.
In such systems, three coupled magnetic coils are positioned equidistantly around the circumference of the welder, in a plane parallel to the plane of motion. The coupled magnetic coils are electrically connected in either a delta or "Y" connection to essentially form an orbital motor stator component. A triangular armature or rotor component separately formed as a single body of magnetic material is positioned centrally relative to the stator component, such that each face of the triangular armature is adjacent to a magnetic coil. The armature is maintained in the horizontal orbital plane by a flexural spring support system connected to a massive stationary frame. Orbital motion of the armature results from the application of a controlled three-phase AC current to the coupled magnetic coils, producing force on the armature proportional to the flux generated. This armature motion can be resolved into displacement, velocity, and acceleration vectors proportional in amplitude to the sine and cosine of twice the AC power line frequency. Linear motion of the armature is produce by the addition of a second orbital motor or by splitting each coupled magnetic coil of a single orbital motor into two sections and selectively applying current to various sections in either parallel or series combinations.
Several disadvantages arise from producing orbital motion using coupled magnetic coils. First, employing coupled magnetic coils reduces the system's overall performance, as force generated in one direction always generates counteracting force elements in the opposite direction due to the coupling of the flux paths in the magnetic circuit. Second, the system is incapable of producing motion which is neither orbital nor linear, i.e. pure arbitrary motion. It is desirable to produce arbitrary motion of the welding components when the system needs to compensate for uneven mass distribution of the armature, or when random orbits are desired.
Finally, the control systems for producing the orbital or linear motion of the armature become complex. When coupled magnetic coils are used in an orbital motor, the magnetic flux within the system is constrained to sum to zero. If in addition, the AC phase currents are also constrained to sum to zero, there are not enough degrees of freedom in the magnetic system to generate the arbitrary forces for producing arbitrary motion. If, however, the phase currents are not constrained to sum to zero, enough degrees of freedom exist in the magnetic system to produce the arbitrary forces, but a continuous flux operation is required to generate these arbitrary forces.
The flux across each air gap between the magnetic coils and the adjacent faces of the armature in such systems is a function of all three phase currents and the non-linear magnetics. At no time is there an unused or unenergized magnetic coil. This limits the ability to use any form of fine flux control in such systems.
It is an object of the present invention to overcome these and other limitations of the prior art.